Because the Sun, Moon, stars, and other objects orbited the planet in the cosmology of Middle-earth they were considered part of Arda, leading to Arda sometimes being called the 'Solar System' of Middle-earth. The term for the Earth itself, excluding these celestial bodies, was Ambar or Imbar.

Originally the Earth (Arda) was flat, the continents were surrounded by a mighty ocean (or perhaps by space), Ekkaia or Vaiya, the Encircling Sea. The central and western continents - respectively Middle-earth and Aman - were separated by Belegaer, also called the Great Sea and the Sundering Seas. In the First Age, the area in the northwest of Middle-earth was occupied by the large region of Beleriand, but this was mostly destroyed and submerged in the War of Wrath at the end of that Age.

There was also a separate continent south and east of Middle-earth called the Dark Land. Another separate continent to the east of Middle-earth was the relatively small Land of the Sun, so called because when the world was flat the Gates of the Sun were near there, and it would be scorched. Information regarding both was vague.

Most of Tolkien's stories take place in the north-west region of the continent of Middle-earth, also called the Hither Lands. Other regions included Harad, though "Harad" means "South" and while properly used for the region immediately south of Gondor and Mordor was often loosely used to refer to every land to the south. There were many stretches of sun-scorched desert in Near Harad, but there was supposedly a jungle beyond it which few had ever ventured to, from which the great Oliphaunts were found. Men of Far Harad are also described as being black skinned. The vast lands of the East were collectively referred to as Rhûn, though this may have only referred to those immediately east of the Sea of Rhûn and River Running. The Easterlings, like the Haradrim, were a diverse collection of many races. There was even less knowledge of the extreme eastern end of Middle-earth, where the land was supposed to stop at another sea. It is known that in ancient times the great chain of the Red Mountains ran north to south near here, and was roughly analogous to the Blue Mountains of the west (the world was once symmetrical at the dawn of time). The race of Elves originated near here, though their original home of Cuiviénen no longer existed by the Third Age. It is not known if the Red Mountains survived into the Third Age, either intact or in some reduced form (like the Blue Mountains). It is said that the Blue Wizards went far into the east of Rhûn and beyond to stir up rebellions against Sauron in the lands he held sway over, though the tales of these struggles never reached the west in detail.

In the Second Age, Númenor was raised in the Great Sea for the Edain. This island existed through most of the Second Age, but was destroyed as a result of the pride of the Númenórean people in defying the Ban of the Valar and sailing to Aman in the west.

After the destruction of Númenor, Arda was made round. Aman was taken out of the world, and could only be reached by the Elves, following the straight road that was granted to them. As Aman was taken away from Arda, new lands and continents were created east and west of Middle-earth.

Tolkien derived the Quenya word Arda from prehistoric Elvish *gardā, Quenya regularly losing initial G and shortening long final vowels. The term basically meant "any more or less bounded or defined place, a region". Its use as a proper name for the World was due to ValarinAþāraphelūn".[1] This word from Valarin was said to mean appointed dwelling, referring to the Earth as the abode of the Valar and/or Elves and Men.[2] A common noun arda "realm" turns up in Tolkien's writings on Elvish from the 1930s,[3] but there is not yet any hint that he conceived it as a Quenya name of the world. This seems to be a later usage in the real and the imaginary timeline alike.

In Tolkien's conception, Arda (Earth), was created specifically as "the Habitation" (Imbar or Ambar) for the Children of Ilúvatar (Elves and Men). It is envisaged in a flat Earth cosmology, with the sun, moon and stars revolving around it. Tolkien's sketches show a disc-like face for the world which looked up to the stars.
However, Tolkien's legendarium addresses the spherical Earth paradigm by depicting a catastrophic transition from a flat to a spherical world, in which Aman was removed "from the circles of the world".

The Akallabêth says that the Númenóreans who survived the catastrophe sailed as far west as they could in search of their ancient home, but their travels only brought them around the world back to their starting points. Hence, before the end of the Second Age, the transition from "flat Earth" to "round Earth" had been completed. New lands were also created in the west, analogous to the New World.

Tolkien wrote that Middle-earth or Endor originally conformed to a largely symmetrical scheme that was marred by Melkor. The various conflicts with Melkor (as well as by the action of the Valar on one occasion) resulted in the shapes of the lands being distorted.

Originally, Arda began as a single flat world and the Valar created two lamps to illuminate it, Illuin and Ormal. The Vala Aulë forged great towers, Helcar in the furthest north, and Ringil in the deepest south. Illuin was set upon Helcar and Ormal upon Ringil. In the middle, where the light of the lamps mingled, the Valar dwelt at the island of Almaren in the midst of a Great Lake.
When Melkor destroyed the Lamps of the Valar, two vast inland seas (Helcar and Ringil) and two major seas (Belegaer and the Eastern Sea) were created, but Almaren and its lake were destroyed.

The Valar left Middle-earth and went to the newly formed continent of Aman in the west, where they created their home called Valinor. To discourage Melkor from assailing Aman, they thrust the continent of Middle-earth to the east, thus widening Belegaer at its middle and raising five major mountain ranges in Middle-earth which adopted a relatively symmetrical distribution, namely the Blue, Red, Grey, Yellow Mountains and the Mountains of the Wind. This act, however, ruined the symmetry of the shape of the continents and their intervening seas.

A few years after publishing The Lord of the Rings, in a note associated with the unique narrative story "Athrabeth Finrod ah Andreth" (which is said to occur in Beleriand during the War of the Jewels), Tolkien equated Arda with the Solar System; because Arda by this point consisted of more than one heavenly body (Valinor being another planet and the Sun and Moon being celestial objects in their own right and not objects orbiting the Earth).

The original world as built by the Valar is called Arda Unmarred (Quenya, Arda Alahasta). This world was effectively destroyed by the wars of the Valar and Melkor, and only Valinor still resembled it in part. Arda Unmarred was a flat symmetrical world with everlasting light.

Arda Marred (Quenya, Arda Hastaina) is the name given to the world as it is: the world after the wars of the Valar and Melkor, and the dispersing of Melkor's fëa in the entire world. It is this world from which are formed the hröa of the Children of Ilúvatar, and therefore it is at times a cruel and evil world with plagues, extreme colds, heat, and other concepts which do not exist in Arda Unmarred.

Arda Marred also broke the design of Elvish immortality: in Arda Marred, Elves slowly fade, until at last they are naught but wraiths. Only in Valinor was this fading delayed, which is one reason all Elves had no choice but to go to Valinor in the end. One of the special abilities of the Rings of Power was that they could delay time, and as such were used by Elrond and Galadriel to preserve their realms. After the loss of the One Ring, the Elves had no choice left, and therefore in the Fourth Age, the last Eldar either left for Valinor or faded to shadows of their former selves.

After the Dagor Dagorath, a new world will then be created, which will be Arda Healed (Quenya, Arda Envinyanta). This world will resemble Arda Unmarred, except superior, since it will also incorporate all the good aspects of Arda Marred.

Galadriel's words while parting with Treebeard at Isengard in The Return of the King are "Not in Middle-earth, nor until the lands that lie under the wave are lifted up again. Then in the willow-meads of Tasarinan we may meet in the spring. Farewell!". This might imply that Beleriand will be recovered after the Dagor Dagorath. It is however unknown, since Arda Healed will contain all the good things of Arda Marred, whether Númenor will also be recovered or not.